1. Field of the Invention
This invention relates to an electrophotographic process, and more particularly, to an electrophoto-graphic process for producing multi-color images by utilizing the photomemory effect of a photosensitive material in which titanium dioxide is used.
2. Description of the Prior Art
As electrophotographic process, the so-called Carlson's process has been best known hitherto. The image-forming step of this process fundamentally comprises a charging step for giving electric charge to the surface of photosensitive layer, a subsequent exposing step for exposing the photosensitive layer to an optical image, thereby forming an electrostatic latent image, and then a developing step for converting the electrostatic latent image to a toner image. From the practical point of view, this process is roughly classified into the so-called PPC method involving the step of transferring a toner image onto usual paper and the so-called CPC method in which a toner image is formed on a photosensitive material.
This Carlson's process is markedly distributed currently, particularly in the field of copying systems for monochromatic images. At the same time, application of this process to color copying or color printing for producing a multi-color image by successively repeating the step of reproduction of color images is also being developed. However, many problems must be solved before its practical application. For example, the photosensitivity at the time of exposure tends to be markedly affected by the charging conditions before the exposure. Further, the dark decay of surface charge in the period from charging to development cannot be avoided. Particularly, the dark decay in the nonexposed areas in the course of exposure is quite difficult to avoid because of the nature of image-producing system, and this limits the production of a multi-color image by combining the charging step by a scanning method with the step of exposing to an optical image in the static state or by carrying out the exposure to an optical image by scanning of laser light requiring a long period of time for the exposure. Further, when a film original is contacted with a photosensitive layer and exposed to an optical image, disturbance readily takes place in the electrostatic latent image at the time of peeling off the film after the exposure, so that the reduction of the quality of image is unavoidable. Further, if a photosensitive material containing titanium dioxide as photoconductive material is employed in Carlson's process, an image having high contrast is difficult to produce in contrast with good continuous degradation of the image.
The present inventors have conducted various studies with the aim of solving the above-mentioned problems, in the course of which the inventors have examined the application of the so-called persistent conductivity phenomenon, i.e. the formation of an electrostatic latent image by utilizing the photomemory effect of a photoconductive material. In case of producing an electrostatic latent image by using an N-type semi-conductor, such as titanium dioxide, as the photoconductive material, it is conventional to apply, first, negative charging and then exposure to an optical image, as is well known. However, in the above-mentioned system utilizing the photomemory effect wherein the charging is carried out after the exposure to an optical image, the photomemory is readily erased and an electrostatic latent image is substantially difficult to form if the photosensitive material using titanium dioxide is first exposed to an optical image and then negative charging is carried out. Nevertheless, it has been surprisingly and unexpectedly found that if positive charging is carried out after the exposure to an optical image an electrostatic latent image corresponding to the exposure to an optical image can be formed without the photomemory being erased. Based on this finding, the inventors have conducted additional studies to accomplish this invention.